The present invention relates to a mass spectrometer and a method of mass spectrometry. The preferred embodiment relates to an ion detector system and method of detecting ions.
It is known to use Time to Digital Converters (“TDC”) and Analogue to Digital Converters (“ADC”) as part of data recording electronics for many analytical instruments including Time of Flight mass spectrometers.
Time of Flight instruments incorporating Time to Digital Converters are known wherein signals resulting from ions arriving at an ion detector are recorded. Signals which satisfy defined detection criteria are recorded as a single binary value and are associated with a particular arrival time relative to a trigger event. A fixed amplitude threshold may be used to trigger recording of an ion arrival event. Ion arrival events which are subsequently recorded resulting from subsequent trigger events are combined to form a histogram of ion arrival events. The histogram of ion arrival events is then presented as a spectrum for further processing. Time to Digital Converters have the advantage of being able to detect relatively weak signals so long as the probability of multiple ions arriving at the ion detector in close temporal proximity remains relatively low. One disadvantage of Time to Digital Converters is that once an ion event has been recorded then there is a significant time interval or dead-time following the ion arrival event during which time no further ion arrival events can be recorded.
Another important disadvantage of Time to Digital Converters is that they are unable to distinguish between a signal resulting from the arrival of a single ion at the ion detector and a signal resulting from the simultaneous arrival of multiple ions at the ion detector. This is due to the fact that the signal will only cross the threshold once irrespective of whether a single ion arrived at the ion detector or whether multiple ions arrived simultaneously at the ion detector. Both situations result in only a single ion arrival event being recorded.
At relatively high signal intensities the above mentioned disadvantages coupled with the problem of dead-time effects will result in a significant number of ion arrival events failing to be recorded and/or an incorrect number of ions being recorded. This will result in an inaccurate representation of the signal intensity and an inaccurate measurement of the ion arrival time. These effects have the result of limiting the dynamic range of the ion detector system.
Time of Flight instruments which incorporate Analogue to Digital Converters are also known. An Analogue to Digital Converter is arranged to digitise signals resulting from ions arriving at the ion detector relative to a trigger event. The digitised signals resulting from subsequent trigger events are summed or averaged to produce a spectrum for further processing. A known signal averager is capable of digitising the output from ion detector electronics at a frequency of 3-4 GHz with eight or ten bit intensity resolution.
One advantage of using an Analogue to Digital Converter as part of an ion detector system is that multiple ions which arrive substantially simultaneously at an ion detector and at relatively high signal intensities can be recorded without the ion detector suffering from distortion or saturation effects. However, the detection of low intensity signals is generally limited by electronic noise from the digitiser electronics, the ion detector and the amplifier system. The problem of electronic noise also effectively limits the dynamic range of the ion detector system.
Another disadvantage of using an Analogue to Digital Converter as part of an ion detector system (as opposed to using a Time to Digital Converter as part of the ion detector system) is that the analogue width of the signal generated by a single ion adds to the width of the ion arrival envelope for a particular mass to charge value in the final spectrum. In the case of a Time to Digital Converter, only ion arrival times are recorded and hence the width of mass peaks in the final spectrum is determined only by the spread in ion arrival times for each mass peak and by variation in the voltage pulse height produced by an ion arrival relative to the signal threshold.
It is known to attempt to extend the dynamic range of both Time to Digital Converter based ion detector systems and Analogue to Digital Converter based ion detector systems by switching the transmission of the spectrometer prior to the ion detector. However, these methods have the disadvantage of having a reduced duty cycle.
Another way of attempting to extend the dynamic range of both Time to Digital Converter and Analogue to Digital Converter based ion detector systems is to use an ion detector having multiple anodes which are different sizes. However, such an approach is difficult to implement and the ion detector system can suffer from cross-talk between the anodes.
A method of increasing the dynamic range of a transient recorder by using two Analogue to Digital Converters is known. A transient signal from an ion detector is amplified using two amplifiers having different gains. The two transients are digitized and the digitized data is combined on a time sample by time sample basis. High gain samples are used unless saturation is determined to occur at which point low gain data is substituted. The low gain data is scaled by the difference in gain between the two amplifiers. The result is a combined transient having a higher dynamic range than that obtainable using a single Analogue to Digital Converter. The combined transient is added to other transients which were collected previously using a known averager method. Once a preset number of transients have been averaged the resulting spectrum is stored to disk.
There are, however, certain disadvantages inherent with the known technique. Any errors in the gain of the amplifiers of the Analogue to Digital Converter input stages or DC offsets (amplifier or Analogue to Digital Converter) or signal synchronisation of the Analogue to Digital Converters relative to the trigger event can result in significant shifts in arrival time when the data from both Analogue to Digital Converters is combined. Synchronisation between the two signals presented to the Analogue to Digital Converters is difficult to achieve at high frequencies of digitisation and attempts at correcting any time differences in the signal being digitised is, in effect, limited to one digitisation time interval which may be too coarse to be of any particular use.
The known method also suffers from the same problems as a standard averaging Analogue to Digital Converter system in terms of reduced dynamic range due to the averaging of noise at low signal intensities and degraded resolution due to the digitization of the analogue ion peak width.
Detectors using a combination of both Time to Digital Converter electronics and Analogue to Digital Converter electronics have been employed in an attempt to take advantage of the characteristics of each different type of recording device thereby attempting to increase the dynamic range and the observed time or mass resolution. However, such systems are relatively complex to calibrate and operate. Such systems are also comparatively expensive.
Recent improvements in the speed of digital processing devices have allowed the production of ion detection systems which seek to exploit the various different advantageous features of both Time to Digital Converter systems and Analogue to Digital Converter systems. Digitised transient signals are converted into arrival time and intensity pairs. The arrival time and intensity pairs from each transient are combined over a scan period into a mass spectrum. Each mass spectrum may comprise tens of thousands of transients. The resulting spectrum has the advantage in terms of resolution of Time to Digital Converter systems (i.e. the analogue peak width of an ion arrival does not contribute significantly to the final peak width of the spectrum). Furthermore, the system is able to record signal intensities which result from multiple simultaneous ion arrival events of the Analogue to Digital Converter. In addition, discrimination against electronic noise during detection of the individual time or mass intensity pairs virtually eliminates any electronic noise which would otherwise be present in the averaged data thereby increasing the dynamic range. However, although this technique does represent an improvement over previous known methods, it still suffers from a relatively limited dynamic range and at higher signal intensities it continues to suffer from saturation effects. In addition, it is difficult using the known method to know with any certainty whether the signal has at any time during the acquisition saturated the Analogue to Digital Converter especially if the input signal changes significantly, in intensity during the time during which individual transients are being combined or integrated into a final spectrum (sometimes referred to as the scan time). This can lead to mass accuracy and quantitation errors which are difficult to detect and correct.
It is therefore desired to provide an improved ion detector system and an improved method of detecting ions.